Patent Document 1 discloses a method for producing 3-methyl-2-thiophenecarboxylic acid which comprises reacting a 3,4-dihalobutan-2-one with a thioglycolic acid in the presence of a base. However, there are problems with industrial application of this method such as the use of malodorous thioglycolic acid.
Non-patent Document 1 discloses a method for producing 3-methyl-2-thiophenecarboxylic acid which comprises brominating 3-methylthiophene to 2-bromo-3-methylthiophene, preparing the Grignard reagent of 2-bromo-3-methylthiophene and reacting the Grignard reagent with carbon dioxide. However, this method cannot give 3-methyl-2-thiophenecarboxylic acid in sufficient yields.